Sr0.02La0.98Nb1-xTaxO4 as a novel, chemically stable proton conducting electrolyte

Sr0.02La0.98Nb0.6Ta0.4O4 proton conductor oxide was prepared by auto-combustion and coprecipitation methods from an aqueous solution using citrates and oxalates as complexing agents. After calcinations at 1100 °C the two methods lead to powders with sub-micrometric average particle size, 0.5 μm and 0.7 μm respectively. Upon sintering at 1600 °C, pellets with an average grain size of 11 μm for the auto-combustion method and of 7 μm for the co-precipitation method were obtained. The electrical conductivity measured on sintered pellets at 800 °C in wet argon atmosphere was 1.9×10-4 Scm-1 for the material obtained by co-precipitation method, 1.8×10-4 Scm-1 for the one derived from auto-combustion method and 7.4×10-4 Scm-5 for the one produced by solid state reaction. The improvement in conductivity of the wet chemistry methods with respect to the solid state reaction was ascribed to the higher control of the cationconcentration with the synthetic conditions. Sr0.02La0.98Nb1-xTaxO4 (SLNT, with x= 0.1, 0.2, 0.4) proton conducting oxides were synthesized by solid state reaction for application as electrolyte in solid oxide fuel cells operating below 600 °C. Dense pellets were obtained after sintering at 1600 °C for 5 h achieving a larger average grain size with increasing the tantalum content. Dilatometric measurements were used to obtain the SLNT expansion coefficient as a function of tantalum content (x), and it was found that the phase transition temperature increased with increasing the tantalum content, being T = 561, 634 and 802 °C for x= 0.1, 0.2 and 0.4, respectively. The electrical conductivity of SLNT was measured by electrochemical impedance spectroscopy as a function of temperature and tantalum concentration under wet (pH2O~ 0.03 atm) Ar atmosphere. At each temperature, the conductivity decreased with increasing the tantalum content, being at 600 °C 2.68×10-4, 3.14×10-5, and 5.41×10-6 Scm-1 for the x= 0.1, 0.2, and 0.4 compositions, respectively. SLNT with x= 0.2 shows a good compromise between proton conductivity avoiding the detrimental phase transition for application as electrolyte below 600 °C.